This invention relates to testing equipment for printed circuit boards or the like, and more particularly to a hand-held capacitance probe for continuity/shorts testing of interconnections at a face of boards having multiple conductors.
Printed (or etched) circuit boards are tested after manufacture of the board has been completed but before components are attached and assembly of the module is completed. The type of testing is primarily directed to determining whether or not all of the conductive paths are in their designated positions and no conductors are unintentionally shorted, and whether electrical continuity exists where it should. Various functional tests may be performed after the module is assembled using the external connectors (those included for operation in the end equipment), and also visual inspection can provide an indication of the quality of the part. However, such methods can be time-consuming and expensive, and in any event are not always effective in discovering shorts or opens which may be hidden form view or not exercised in the electrical functional tests chosen. If a circuit board has faults, it is always much cheaper to discover these prior to completing assembly of a module. For these reasons, various test mechanisms have been proposed which perform the function of scanning the surface of a circuit board and providing an indication of the wiring integrity.
The most widely used testing methods for production type testing of circuit boards make use of so-called bed-of-nails probes, where an array of closely-spaced pins is used to make contact with conductors on the board, or an X-Y prober along with a multimeter or capacitance meter. Also, continuous motion test equipment using X-Y motion systems and specialized position detection are available. These prior systems are very costly and cumbersome, and most require extensive software to customize applications. Because of the large number of diverse circuit boards, and the small conductor sizes now used, these prior methods have not provided viable solutions to the fast turn-around testing needed today.
In copending application Ser. No. 640,553, filed Jan. 14, 1991 (continuation of application Ser. No. 424,654, filed Oct. 20, 1989), by Ellsworth W. Stearns, now U.S. Pat. No. 5,138,266, a testing device is disclosed which uses a single probe charge measuring technique, where the amount of charge transferred when the probe contacts a node is detected.
In U.S. Pat. No. 5,017,863, issued from application Ser. No. 424,396, filed Oct. 20, 1989), by Richard I Mellitz, a testing device is disclosed which uses a laser beam impinging upon an electro-emissive grid to charge nodes of a printed wiring board, and this charge is detected to provide an indication of continuity or shorts.
In U.S. Pat. No. 4,229,693 a single-probe capacitance measurement method is described, used in testing of printed circuit boards. A probe is used to make contact to the conductors of the board to measure the capacitance to thereby detect open circuit and short circuit networks on the board. This technique, however, does not provide any facility for measuring any previous electrical states of the network being probed. Thus, when two or more networks have been identified as open or shorted, they must be located and verified with two probes using a resistance type of measurement. Further, this method requires that the capacitance values of each network be known. In order to acquire this data, a known good module must be available to obtain the capacitance values since they cannot easily be calculated, or a group of unknown modules may be used and the results analyzed to obtain the values. Commercial equipment available using this type of testing method includes a Teledyne TAC capacitance probing system.
Presently the lowest cost approach to verifying opens and shorts is a hand-held ohm meter. This is not a viable option for present-day printed wiring boards due to the large number and small size of the circuits on such boards.